Requirements for a fuel economy engine in the field of a vehicle underlie a recent campaign to combat the global warming problem and energy problem on a worldwide scale. A lean burn engine is one of the most conspicuous examples. Especially in the cylinder injection engine, a fuel is directly injected in the combustion chamber, gas mixture is stratified and combustion is carried out at an air-fuel ratio of 40 or more. This arrangement reduces the fuel cost, as compared with the prior art inlet port injection engine. In the meantime, solution to environmental problems including air pollution requires improvement of the performance of an emission gas, and tighter emission control is being enforced in each country of the world every year.
The three-way component catalyst having been used in the prior art fails to clean the NOx discharged from the engine (combustion chamber) in the lean operation mode. So in the lean burn engine, a lean NOx catalyst is commonly provided in an emission path to clean NOx in the lean operation mode. The lean NOx catalyst stores NOx in the lean operation mode, namely, in the atmosphere of oxidation, and HC and CO are cleaned by oxidation. Further, if rich operation (hereinafter referred to as “rich spike” or “rich control”) is started, namely, the atmosphere of reduction is formed in order to clean the stored NOx, then the stored NOx is desorbed to cause oxidation/reduction reaction, together with the reducing agent (H2, CO, HC) discharged from the combustion chamber. Thus, the lean NOx catalyst is effective in reducing the amount of NOx emission, whereas it requires the performance of the catalyst (NOx storage capacity) to be diagnosed.
In the process of diagnosing the performance of lean NOx catalyst (NOx storage capacity), unexpected deterioration of combustion unexpected fluctuation in the amount of NOx emission from the engine (combustion chamber) causes deterioration of diagnostic accuracy of the catalyst. Especially in the case of a cylinder injection engine, the combustion injection valve is installed in the combustion chamber. This makes it easier for the cinders called the deposit to stick to the injection port of the fuel injection valve. Thus, fluctuations in the shape of spray at the time of fuel injection may be caused by the deposit. The fluctuations in the shape of spray often give an adverse effect to the combustion performance, especially to the emission gas performance in the stratified lean operation mode, resulting in a big change in the amount of NOx discharged from the combustion chamber. Further, when the EGR rate in the lean operation mode may not reach the required level due to a trouble of the EGR valve or the like, fluctuations in the amount of NOx flowing into the lean NOx catalyst may be caused.
For example, Japanese Patent Laid-open No. 2000-337130 (pages 1 and 2, FIGS. 1 through 11), wherein an oxygen sensor is arranged downstream from the lean NOx catalyst in the emission path, a technique of estimating the amount of NOx stored in the lean NOx catalyst in the lean operation mode used prior to rich spike, based on the rich reverse delay time of the sensor in the rich spike mode. This technique is to diagnose the performance (the maximum NOx storage volume) of the lean NOx catalyst, based on the amount of the stored NOx. According to this method, however, when there is an unexpected reduction in the amount of NOx flowing into the lean NOx catalyst for the aforementioned causes and others, the amount of the stored NOx estimated in the rich spike mode also reduces, even if the lean NOx catalyst is not subjected to deterioration. This may lead to such an incorrect diagnosis that the lean NOx catalyst has deteriorated.
In Japanese Patent Laid-open No. Hei 07-180535 (pages 1 through 5, FIGS. 1 and 2), an NOx sensor for detecting the concentration is arranged downstream from the lean NOx catalyst, and is used to detect the amount of NOx flowing out of the lean NOx catalyst without being stored in that NOx catalyst in the lean operation mode. When the amount of unstored NOx has been detected to increase in excess of a predetermined value, the lean NOx catalyst is evaluated to have deteriorated. According to this method, however, if there has been an unexpected increase in the amount of NOx discharged from the combustion chamber or the amount of NOx flowing into the lean NOx catalyst in the lean operation mode due to the aforementioned causes, this may lead to such an incorrect diagnosis that the lean NOx catalyst has deteriorated, due to an increase in the amount of unstored NOx in the lean operation mode, even if there is deterioration in the lean NOx catalyst (reduction in NOx storage capacity).
Further, in Japanese Patent Laid-open No. 2002-266629 (pages 1 through 18, FIGS. 1 through 15), at least one of the maximum amount of NOx absorbed by the catalyst and the NOx absorption speed of the catalyst is calculated using the output of a sensor, arranged downstream from the lean NOx catalyst, for detecting the NOx concentration. When the maximum amount of NOx absorbed by the catalyst or the NOx absorption speed of the catalyst calculated in the aforementioned manner is smaller than the evaluation value, the catalyst is evaluated to have deteriorated. This proposal, however, may lead to such an incorrect diagnosis that the lean NOx catalyst has deteriorated, when there has been an unexpected increase in the amount of NOx discharged from the combustion chamber or the amount of NOx flowing into the lean NOx catalyst in the lean operation mode due to the aforementioned causes.
As described above, according to the prior art, when there has been an unexpected increase in the amount of NOx discharged from the combustion chamber or the amount of NOx flowing into the lean NOx catalyst, the lean NOx catalyst may be evaluated as having deteriorated, even if there is no deterioration in the lean NOx catalyst.